Aspects and embodiments of the invention are directed to Autonomous Underwater Vehicles (AUVs) and methods for controlling an AUV; more particularly, AUVs equipped and enabled to operate in a hover mode and methods for controlling the operational hover mode as well as its engagement and disengagement.
Autonomous Underwater Vehicles (AUVs) are well developed and used in numerous subsea applications, most notably collecting bathymetric data and ocean bottom imaging by means of sensors carried aboard. As the name implies, these vehicles operate autonomously without pilots, unlike Remotely Operated Vehicles (ROVs), which typically include a coupled tether management system (TMS) as known in the art. Although AUVs may have an acoustic communication mechanism for communication with a separated, distant launch/retrieval platform, for the most part a flight plan is loaded into an onboard computer, the vehicle (AUV) is lowered into the water, and it flies that flight plan usually following the marine bottom closely to collecting data of interest.
AUVs generally carry their own onboard power source such as batteries or fuel cells, further differentiating them from ROVs. The onboard power source must power both the vehicle's propulsion as well as the onboard instrumentation. To minimize the power used for propulsion and maximize battery life and mission duration almost all AUVs take the shape of a torpedo, as illustrated in FIG. 1. This torpedo shape minimizes the vehicle's drag in the water.
Depending on the mission and depth rating of the AUV, its length, girth, and weight can range quite broadly. Shallow rated vehicles, e.g., may be as small as 6 feet in length, 10 inches in diameter, and weigh only a few hundred pounds (e.g., Bluefin Robotics). AUVs depth rated to 10,000 feet or more may commonly be as long as 21 feet, 40 inches in diameter, and weigh between 2000 and 3300 pounds in the air (e.g., Kongsberg; C&C Technologies). As known, all AUVs are made near neutrally buoyant in the water by use of syntactic foam or other recognized means.
The aforementioned torpedo design conserves power, allowing for longer missions given a fixed amount of power. AUVs of this design are typically capable of two-day long missions, although advances in battery and fuel cell technology promise longer duration missions in the future. Of these torpedo-like AUVs, there are some generally shared features amongst all of them. For example, propulsion derives from a variable speed propeller driven by an electric motor near or at the rear of the vehicle (FIG. 1). Flight surfaces, e.g., an elevator and rudder are generally mounted just forward of the propeller and may be replicated in whole or in part farther forward on the vehicle, depending on the mission. In some cases, flight surfaces may be absent and the main propeller or propulsion system is articulated so as to direct thrust as needed for steering. While there do exist AUVs with thrusters, these are generally smaller, lighter vehicles for shallow water applications such as, e.g., hull inspection. Typical torpedo-like AUVs do not have thrusters but rather depend on hydrostatic forces of water moving over their adjustable flight surfaces (wings) to adjust their heading and/or depth (FIG. 15).
AUVs of this aforementioned style are very mature and capable, and are used in all of the oceans of the world for survey work. Their dependence, however, on the hydrostatic forces of water acting on their flight surfaces or the body of the vessel for control, which is largely derived from their forward motion through the water, ill adapts them for hovering still and controlled hovering in the water column. While horizontal and vertical thrusters could be added to the vehicle to add this capability, this substantially adds to the vehicle's complexity and cost as well as adding drag that affects speed and/or mission duration given fixed battery life. Those skilled in the art will appreciate that there are other reasons beyond cost and complexity why even tunnel thrusters are problematical for the applications embodied by the instant invention.
A technique of ocean bottom seismic sensing involves downloading data from a series of Ocean Bottom Seismic nodes (OBS nodes or, simply ‘nodes’). Like the AUV, nodes are autonomous; they contain a self-contained power source and are able to record and store large amounts of seismic, electromagnetic, and similar data recorded by various sensors contained in the node. Data may be communicated with the node via multiple mechanisms; for example, wireless acoustic methods are commercially available, however bandwidth for such systems is very low, even over short distances. A node may have 64 gigabytes or more of recorded data to be recovered by the AUV. Thus, radio frequency (RF) or optical communications remain available as choices. Radio frequencies require very close antenna proximity as sea water is very poor at transmission of radio frequencies, with attenuation in excess of 40 db/meter. Optical transmission can carry very high bandwidths depending on the technology employed but transmission fidelity is adversely affected by the turbidity of the water, which would be made more problematic by AUV thrusters. Operating near a muddy ocean bottom as is found in the Gulf of Mexico, for example, thrusters tend to turn up the mud making optical transmission of data difficult or impossible. Turbidity in the deep ocean is generally not a problem except for the issues created by the vehicle itself. Whether via RF or optical transmission, and even assuming a gigabit/second or better transmission rate, large datasets are difficult to collect by a moving AUV; e.g., in motion a slow fly-by or tight circling pattern.
Although ROVs can hover and could perform the tasks at hand, ROVs require pilots, and while some ROVs can achieve speeds of torpedo-style AUVs, the speed with which ROVs can traverse a large field of widely spaced nodes (which may typically be deployed on a grid of 200-500 meters) is limited not by the ROV's speed but by the speed that the ROV's tether management system (TMS) can be towed through the water.
For these and other reasons known in the art, there exists a need to provide a conventional torpedo-styled, high-speed AUV with the ability to stop and controllably hover with stability over a target on the marine bottom. Advantageously, the ability to stop and hover the AUV would occur without forward speed and without thruster assistance, using the AUV's conventional flight surfaces for attitude, heading, and elevation control.